Method for calibrating automatic gain control in wireless devices

ABSTRACT

Disclosed herein is an iterative process for calibrating an AGC in a wireless system, wherein the iterative process includes transmitting a calibration signal, receiving the calibration signal, decoding the calibration signal to produce a measurement, storing the measurement and changing an AGC gain setting.

PRIORITY CLAIM

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 60/434,848, filed on Dec. 20, 2002, entitled “Method ForCalibrating the Receive AGC In A Multichannel CommunicationsTransceiver,” incorporated herein by reference.

BACKGROUND

The invention disclosed herein relates to wireless networks. Morespecifically, the invention relates to calibrating the automatic gaincontrol (“AGC”) of a receiver in a wireless device.

Wireless local area networks (“WLAN”) allow electronic devices, such ascomputers, to have network connectivity without the use of wires.Network connections may be established via, for example, radio signals.Connections with existing wired networks may be provided by a wirelessaccess point (“AP”) having a wired Internet or Ethernet connection andradio communication circuitry capable of transmitting data to andreceiving data from any compatible wireless device. The AP may provideInternet and/or network connectivity to such wireless devices (e.g.,portable computers) called receiver stations (“STA”) by transmitting andreceiving data via radio signals.

Data signals transmitted from an AP to a STA or from a STA to an AP mayvary in quality and strength. For example, an AP and a STA situated inrelatively close proximity may achieve a strong communication signal,whereas an AP and a STA with a relatively large physical distancetherebetween may achieve a weak communication signal. Depending on thequality of the signal received, a STA or an AP may use an AGC to amplifythe received signal to a magnitude and phase suitable foranalog-to-digital conversion. The AGC may automatically adjust theamount of gain based on signal characteristics. For instance, a greatergain may be applied to a weak received signal whereas a lesser gain maybe applied to a strong received signal. In this manner, the AGC canconsistently provide an analog-to-digital converter (“ADC”) withreceived signals of similar or identical strength, regardless of thestrength of the signals received. Such use of an AGC permits use of thefull dynamic range of the ADC.

Regular calibration of the AGC may provide wireless communicationsystems with enhanced performance. That is, it may be desirable todetermine how the signal gain varies for different settings of the AGC.Certain hardware may be implemented in a STA or an AP to accomplish AGCcalibration; however, additional hardware may be unacceptable due to,among other things, an increase in production cost. Thus, a technique tocalibrate the AGC without the use of additional hardware is desirable.

BRIEF SUMMARY

Accordingly, there is disclosed herein a method for calibrating an AGCin a wireless device. In one embodiment, the method comprisestransmitting a calibration signal, receiving the calibration signal,decoding the calibration signal to produce a measurement, storing themeasurement and changing an AGC gain setting.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of the embodiments of the invention,reference will now be made to the accompanying drawings in which:

FIG. 1A illustrates a multiple-input, multiple-output signaling systemblock diagram in accordance with certain embodiments of the invention;

FIG. 1B illustrates a receiver block diagram in accordance with certainembodiments of the invention;

FIG. 2 illustrates a block diagram of the orthogonal frequency divisionmultiplexing technique in accordance with certain embodiments of theinvention;

FIG. 3 illustrates a flow diagram in accordance with certain embodimentsof the invention; and

FIG. 4 illustrates a second flow diagram in accordance with certainembodiments of the invention.

NOTATION AND NOMENCLATURE

Certain terms are used throughout the following description and claimsto refer to particular system components. As one skilled in the art willappreciate, companies may refer to a component by different names. Thisdocument does not intend to distinguish between components that differin name but not function. In the following discussion and in the claims,the terms “including” and “comprising” are used in an open-endedfashion, and thus should be interpreted to mean “including, but notlimited to . . . ” Also, the term “couple” or “couples” is intended tomean either an indirect or direct electrical connection. Thus, if afirst device couples to a second device, that connection may be througha direct electrical connection, or through an indirect electricalconnection via other devices and connections.

DETAILED DESCRIPTION

The following discussion is directed to various embodiments of theinvention. Although one or more of these embodiments may be preferred,the embodiments disclosed should not be interpreted, or otherwise used,as limiting the scope of the disclosure, including the claims. Inaddition, one skilled in the art will understand that the followingdescription has broad application, and the discussion of any embodimentis meant only to be exemplary of that embodiment, and not intended tointimate that the scope of the disclosure, including the claims, islimited to that embodiment. The embodiments described herein may beprovided in the context of IEEE 802.11 compliant devices, but otherwireless protocols, now known or later developed, may be used as well.Furthermore, the embodiments described herein may be implemented in thecontext of multiple-input, multiple-output (“MIMO”) signaling systems,but other signal systems also may be used.

In a MIMO signaling system, the rate at which data is transferred (“datarate”) between an access point (“AP”) and a wireless device (“STA”) maybe raised by increasing the number of antennas and transmitter/receivermodules (“transceivers”) associated with each wireless device in thesystem. For instance, a system having an AP with multiple antennas andtransceivers and an STA with multiple antennas and transceivers may havea higher data rate than a system having an AP with a singleantenna/transceiver and an STA with a single antenna/transceiver. Themultiple antennas and transceivers are part of a design that attempts toachieve a linear increase in data rate as the number of transmitting andreceiving antennas increases. The calibration technique described hereinmay be applied to any wireless MIMO system.

The subject matter disclosed below provides a technique for calibratingan automatic gain control (“AGC”) in a MIMO system without the use ofadditional hardware. Referring to FIG. 1A, a MIMO STA 102 may havetransceivers 104, each having an associated antenna 106. Some or all ofantennas 106 may be mutually coupled to each other by way of a wirelesschannel 110. Each transceiver 104 may include an AGC 108 for regulatingthe strength of received signals. In at least one embodiment, atransceiver 104 in STA 102 may transmit a predetermined signal by way ofthe wireless channel 110 to the other transceivers in the STA 102. Thereceiving transceivers 104 may measure the magnitude and phase ofsignals received for each possible gain setting of the AGCs 108. Thetransceivers 104 may each have a memory 112 for recording thesemeasurements for later use. In this manner, the AGCs 108 of thereceiving transceivers 104 may be calibrated. The transmittingtransceiver may then take a turn as a receiving transceiver while one ofthe calibrated transceivers transmits the predetermined signal forcalibrating the AGC 108.

FIG. 1B is a block diagram of an illustrative STA having a transmittingtransceiver 104A in communications with a receiving section of a secondtransceiver 104B by way of a channel 110. A receiver section oftransceiver 104B may comprise, among other things, a preselector filter114; a radio frequency amplifier 116 with adjustable gain; an imagerejection filter 118; a high frequency oscillator 120; a mixer 122; anintermediate frequency bandpass filter 124; an intermediate frequencyamplifier 126 with adjustable gain; a second image rejection filter 128;an intermediate frequency oscillator 130; a phase delay 132;intermediate frequency mixers 134 and 136; low pass filters 138 and 140;baseband amplifiers with adjustable gain 142 and 144; analog-to-digitalconverters (“ADCs”) 146 and 148; a digital processing device 150; and amemory 152. The digital processing device 150 may implement an adaptivegain control 154, a digital equalizer 156, a synchronization module 158,a cyclic prefix dropping module 160, a Fast Fourier Transform (“FFT”)module 162, a frequency domain filter 164, and subsequent modules fordemodulating received signals.

Preselector filter 114 screens out signals outside the design range oftransceiver 104B. Adjustable gain amplifiers 116, 126, 142 and 144provide gain to the receive signal at various points in the receiverchain. Mixers 122, 134, and 136 are preceded by image rejection filters118 and 128 to reject the image frequency bands and the noise andinterference present therein. Mixers 122, 134, and 136, along with thesubsequent filters 124, 138 and 140, provide frequency down-conversion,thereby shifting the carrier frequency of the receive signal first to afixed intermediate frequency, and then to baseband. The baseband signalhas both in-phase and quadrature components that are converted todigital receive signals by the ADCs 146 and 148.

The digital processing device 150 accepts the digital receive signalsfrom the ADCs 146 and 148. The digital processing device 150 operates onthe digital signals to extract the receive data. As part of itsoperations, the digital processing device 150 provides digital settingsfor the adjustable gain amplifiers 116, 126, 142, and 144. The gainsettings may be determined by applying an adaptive algorithm to thedigital receive signals. Alternatively, the gain settings may be set bythe processing device in accordance with preprogrammed criteria. Ineither event, the gain settings affect the amplitude and phase of thedigital receive signals in a manner that drifts over time. Gain errorsof 0.1 dB or greater are common at each gain amplifier, often resultingin a cumulative receiving transceiver gain error of approximately 1 dBor greater. Adjustments to the gain settings during operation will alsosignificantly alter the amplitude and phase of the digital receivesignals. These alterations may adversely affect the performance of thedigital equalizer 156 and the frequency domain filter 164. Accordingly,it is desirable to periodically measure the effects of the gain settingsto permit accurate compensation in the receiver chain.

The ADCs 146 and 148 may accept as inputs a continuously varying analogdata signal and a clock signal provided by any suitable entity. The ADCs146 and 148 may sample the incoming analog data signal with each clocktick and output the sample in digital form. Provided with a continuousanalog data signal, the ADCs 146 and 148 may output a stream of digitalvalues (i.e., a “sample stream”), each value representing the voltage ofthe incoming analog data signal at a clock tick. These output values maybe in the form of digital inphase (I) and quadrature (Q) outputs, whichlater may be combined to form a complex (i.e., real+j*imaginary) signal.The digital stream output by the ADCs 146 and 148 then may be processedby the digital equalizer 156 and the synchronization block 158. Anycyclic prefix that may have been prepended to the signal may be droppedat the cyclic prefix drop 160. The digital output subsequently may beconverted from time domain to frequency domain by way of the fastFourier transformer 162. The frequency domain filter 164 compensates forthe attenuation and phase shift caused by the channel, but may also beused to compensate for amplitude and phase shifts caused by adjustmentsto the gain settings. The output from the frequency domain filter 164can then be decoded and/or descrambled as appropriate to obtain thecommunicated data, which can then be provided to the intended recipientapplication.

The number of bits used in the digital representation of input samplesmay vary from one ADC to another. For example, 10-bit ADCs may dividethe input voltage range into 2¹⁰ (i.e., 1024) voltage steps. Thus, eachdigital output from the 10-bit ADC would be the number between from 0 to1023 that represents the binary value that most closely matches theactual input voltage. It is axiomatic that the converting an analogsignal to digital samples introduces a small amount of error, i.e., thedifference between the actual input voltage and the closest voltagehaving a binary representation. To minimize the effect of these errors,it is desirable to match the range of the analog input voltages to themaximum range of the ADC. Thus, the maximum expected analog inputvoltage should convert to a value near the maximum representable by theADC, and the minimum expected analog input voltage should convert to avalue near the minimum representable by the ADC. This matching causesthe errors to be relatively insignificant with respect to the range ofthe analog signal.

In some transceiver embodiments, the data signals received by theantenna 106 may vary in amplitude from less than 10 microvolts togreater than 10 millivolts. Factors such as the distance between thetransmitting and receiving antennas, physical obstructions, and signalreflections may cause the data signals to vary in strength whiletraveling through the channel 110. Because the ADCs 146 and 148 mayrequire a certain signal strength for proper dynamic range matching, thereceiving transceiver 104B may adjust the incoming data signal tocompensate for the effects of channel 110. For example, in at least someembodiments, the ADCs 146 and 148 may require incoming analog datasignals to range between −1 V and +1 V. Thus, a weak incoming datasignal at a level of 10 microvolts may require a total voltage gain of100,000. Similarly, a strong incoming data signal of 10 mV may require alesser voltage gain of 100. The receiving transceiver 104B may applysuch gains to signals, thereby providing the ADCs 146 and 148 withsignals of suitable strength.

In some cases, the receiving transceiver 104B may attempt to applyinappropriate gains to certain incoming data signals. For example, thereceiving transceiver 104B may attempt to apply a gain of 100,000 to astrong, incoming data signal, providing a signal size exceeding thelimits of internal hardware components or the ADCs 146 and 148 andresulting in severe signal distortion. Similarly, the receivingtransceiver 104B may inappropriately attempt to apply a gain of 100 to aweak, incoming data signal, resulting in a signal size too small to beproperly converted to digital form by the ADCs 146 and 148. The AGCcontroller 154 may correct for such a problem by adjusting the receivingtransceiver 104B gain, thereby keeping the data signal input into theADCs 146 and 148 at an appropriate size.

The AGC controller 154 may be implemented as a hardwired circuit,firmware, or a software routine. In some embodiments of the adjustablegain amplifiers 116, 126, 142, 144, the gain may be adjusted digitallythrough the action of switches that route the receive signal through oneof various available paths. The different paths may have different gainsor attenuation, thus providing the adjustable gain. In otherembodiments, the digital gain setting may be converted into an analogvoltage to be applied to the gain control input of the adjustable gainamplifiers. The AGC controller 154 may determine appropriate gainsettings by applying a control algorithm to the in-phase and quadraturephase digital receive signals. The AGC controller 154 may also receivecommands from the processor 150 to set the gain settings to fixed valuesor to increment/decrement the gain settings.

In one embodiment, signals may be transmitted using the orthogonalfrequency division multiplexing (“OFDM”) modulation. OFDM is amodulation technique for transmitting large amounts of digital data overa radio wave. OFDM uses the computational efficiency of the fast Fouriertransform (“FFT”) and inverse fast Fourier transform (“IFFT”) totransmit multiple channels of data without self-interference. OFDM worksby splitting the radio signal into multiple sub-signals. Each sub-signalmay be represented as a frequency coefficient. The frequencycoefficients are supplied to an inverse fast Fourier transformer,wherein bits of transmission data are distributed among the sub-signals,depending on the signal-to-noise ratio of each sub-signal. Thetransmission data then is transmitted simultaneously at differentfrequencies to the receiver. OFDM reduces the amount of interference insignal transmissions and is used in 802.11a WLAN technology.

FIG. 2 illustrates a block diagram of an OFDM system 200 in which atransmitter 210 is in communications with a receiver 212 by way of awireless channel 214. The transmitter 210 is coupled to aparallel-to-series converter (“PSC”) 206, which in turn is coupled to anInverse Fast Fourier Transformer (“IFFT”) 202. Conversely, the receiver212 is coupled to a serial-to-parallel converter (“SPC”) 208, which inturn is coupled to a Fast Fourier Transformer (“FFT”) 204. Theconstellations C_(i), are visual representations of the possible complexfrequency coefficients that may be supplied to the IFFT 202 and receivedfrom the FFT 204. In FIG. 2, each constellation point represents acorresponding 4-bit data value.

The OFDM method provides the IFFT 202 with sets of complex data valuesfrom various signal constellations C_(i). The IFFT 202 subsequentlytransforms the sets from the frequency domain to the time domain. Oncein the time domain, data values are converted from parallel format toserial format by PSC 206, effectively creating a single stream of datato be transmitted from transmitter 210 to receiver 212 by way of channel214. Once received by receiver 212, the data is converted from serialformat back to parallel format by SPC 208, converted from time domainback to frequency domain by FFT 204 and then provided to dataconstellations C_(i) as constellation data points.

A predetermined training signal is transmitted over channel 214 to allowthe receiver to estimate the frequency response and phase shift added todata signals by the channel 214. The training signal is sent fromtransmitter 210 to receiver 212. The receiver 212 then may measure theincoming training signal and determine the frequency response and phaseshift provided by the channel 214. The frequency response and phaseshift information then may be used to correct channel errors in actualdata signals (i.e., signals carrying data unknown to the receiver 212).In one example, a constellation point C7, when transmitted fromtransmitter 210, is 1<45° (magnitude of one at an angle of forty-fivedegrees). When received at receiver 212, however, the constellationpoint is 0.25<−90°. The receiver 212 may compare the receivedconstellation point value to the original constellation point value anddetermine the frequency response and phase shift provided by the channel214. In the present example, the channel 214 attenuated the data signalby a factor of 4 and shifted the phase of the signal by −135°. Similarchannel estimation data may be determined for each frequencycoefficient. After the training signal, an actual data signal may betransmitted by transmitter 210 over channel 214. In order to recover thedata at the receiver 214, the C7 value output by the FFT 204 ismultiplied by 4 and phase shifted 135°. Thus, the undesirable effects ofchannel 214 on the data signal are negated.

Because changing the gain of the receiver causes changes in themagnitude and phase of the received signals, it is undesirable to changethe gain settings of the receiver 212 between the transmission of thechannel estimation information and the transmission of the actual datasignal. Thus, the AGC controller 154 may set the amplifier 116, 126,142, 144 AGC values before the training signal and hold them unchangedthroughout the channel estimation and data transmission process.

However, in some cases, normal OFDM operation may not be used. That is,the amplifiers 116, 126, 142 and 144 of FIG. 1B may be adjusted duringoperation by the digital processing device (“processor”) 150 through theAGC controller 154. As mentioned above, such adjustments introduceamplitude and phase changes to the digital receive signals, which maycause errors if no corrective action is taken in the processor 150.

Referring now to FIGS. 1B and 3, FIG. 3 provides a flow chart of anillustrative AGC 108 calibration technique. The flow chart is dividedinto three columns, each of which will be discussed in turn. Eventsdescribed in column 1 properly set the transmission power of thetransmitting transceiver 104A of FIG. 1B. The process may begin with theprocessor 150 adjusting the gain setting to the maximum value (block302). The transmitting transceiver 104A may adjust the transmissionpower to the minimum power setting (block 304). In blocks 306-312, thetransmitting transceiver 104A then increments the transmission poweruntil the processor 150 determines that the signal power at the ADCs 146and 148 is at the top of the ADC 146 and 148 dynamic signal power range.Specifically, the transmitting transceiver 104A may transmit apredetermined training signal as previously described (block 306). Thereceiving transceiver 104B subsequently may receive, measure and storein memory 152 a power value for the digital receive signals (block 308).The receiving transceiver 104B then may determine whether the signalpower at the ADCs 146 and 148 is at the top of the ADC 146 and 148dynamic signal power range (block 310). If the signal power at the ADCs146 and 148 is not at the top of the ADC range, the transmittingtransceiver 104A may increase the transmission power a relatively smallamount (block 312) and then may attempt another training signaltransmission (block 306). However, if the signal power at the ADCs 146and 148 is at an appropriate level, then the transmission power is setcorrectly and may not need to be adjusted.

If the transmission power is properly set, then the actual AGC 108calibration process may begin. The AGC 108 calibration process maycomprise an iterative loop wherein calibration measurements may be takenand stored for each possible AGC 108 gain setting. Specifically, asillustrated in column 2, the process may begin with the transmittingtransceiver 104A transmitting an AGC 108 calibration transmission (block314). The AGC 108 calibration transmission may be composed of a singlefrequency centered on an FFT 304 bin and may be generated by applying asingle, non-zero coefficient to the IFFT 302. The receiving transceiver104B then may receive the calibration transmission, decode thecalibration transmission with the FFT 304 and store the amplitude andphase results as a complex number in memory 116 (block 316). Theprocessor 150 may determine whether the gain setting is at the minimumlevel (block 318). If the AGC 108 is not set at the minimum gain level,the gain setting may be decremented by one gain level setting (block320). The iterative AGC 108 calibration process then may continue withthe transceiver 104A transmitting another calibration transmission(block 314). Otherwise, if the AGC 108 is set at the minimum gain level,the receiving transceiver 104B may have stored into memory 152calibration measurements for each possible AGC 108 gain setting,completing the calibration loop.

Because the calibration measurement for any given AGC 108 gain settingis relative to the calibration measurement for any other gain setting,the receiving transceiver 104B may normalize all calibrationmeasurements stored in memory 152 to the calibration measurement of themaximum AGC 108 gain setting (i.e., the first measurement). Column 3 ofFIG. 3 illustrates the normalization process of dividing all storedcalibration measurement values by the first calibration measurementvalue (block 322).

In FFT 304-based operations, a slip of a measurement sample in the timedomain causes an undesirable, progressive phase shift across thefrequency domain outputs. Precisely-timed measurements may preventsample slips. Additionally, in accordance with standard OFDM practices,a relatively small cyclic prefix may be prepended to the AGC 108calibration transmission to account for the non-zero propagation fromthe digital-to-analog converter (“DAC”) in transceiver 104A to the ADCs146 and 148 in the receiving transceiver 104B. The cyclic prefixsubsequently may be removed by the cyclic prefix drop 160 in thereceiving transceiver 104B.

In the embodiments described above, as the gain setting of the AGC 108is decremented, the strength of the signals provided to the ADCs 146 and148 decreases. While tolerable for a relatively small range of AGC 108gain settings, the processor 150 may eventually attenuate the signalssuch that the ADCs 146 and 148 may no longer be capable of performingmeasurements with sufficient accuracy.

An alternative embodiment addresses this problem by decrementing thegain settings of the AGC 108 over a small range of gain in k decrementswhile taking measurements at each gain setting. After k decrements, thetransmission power may be incremented by L dB, an amount sufficient torestore the strength of signals provided to the ADCs 146 and 148. Thevalue of L may be determined by measurement or design of the transceiver104B. The process of k AGC 108 decrements followed by a restoringtransmit power increment may continue in this fashion until the entirecalibration process is complete. However, adjusting the transmissionpower may introduce changes in subsequent calibration measurements, socare should be taken to compensate the calibration measurements for thetransmission power adjustments.

To compensate for adjusting the transmission power, the k^(th)calibration measurements may be performed twice. The first k^(th)calibration measurement, resulting in the complex calibration valuec_(k,1), may be performed immediately prior to increasing thetransmission power. The second k^(th) calibration measurement, resultingin complex calibration value c_(k,2), may be performed immediately afterincreasing the transmission power. For example, if k=10, then the tenthcalibration measurement may be performed twice. The first measurementmay be performed immediately prior to increasing the transmission powerand the second measurement may be performed immediately after increasingthe transmission power. After the increase in transmission power,further AGC 108 calibration measurements may be compensated for thetransmit power increase. In this example, these would be the 11^(th) andsucceeding measurements.

The complex correction factor f_(k) needed to compensate for theincrease in transmit power then may be determined by dividing the firstmeasurement by the second measurement. That is,

$f_{k} = {\frac{c_{k,1}}{c_{k,2}}.}$Thus, for succeeding AGC 108 calibration measurements, each measuredvalue may be multiplied by the correction factor f_(k) to compensate forthe transmission power adjustment. That is,c′ _(i) =c _(i) ·f _(k) for all i>k,where c′_(i) is the corrected complex calibration measurement and c_(i)is the raw complex calibration measurement.

Continuing with this example, k=10 denotes that the transmission poweris increased after every 10^(th) AGC 108 gain decrement. To compensatethe subsequent measured values for the transmit power increases, eachmeasured value may be multiplied by the product of the correctionfactors. That is,

$c_{i}^{\prime} = {\begin{Bmatrix}{c_{i}f_{10}} & {{{for}\mspace{14mu} 10} < i \leq 20} \\{c_{i}f_{10}f_{20}} & {{{for}\mspace{14mu} 20} < i \leq 30} \\{c_{i}f_{10}f_{20}f_{30}} & {{{for}\mspace{14mu} 30} < i \leq 40} \\\; & {{etc}.}\end{Bmatrix}.}$However, instead of multiplying each raw measurement for the AGC 108gain settings by perhaps multiple correction factors (f₁₀·f₂₀ . . . ), amore practical approach comprises multiplying the appropriate factorstogether, thereby forming a combined correction factor g. That is,g=f ₁₀ ·f ₂₀ . . . .Thus, each raw measurement value may be multiplied only by a singlecorrection factor g, which is updated at each k^(th) step by beingmultiplied with a new correction factor f_(k).

Divided into three columns, FIG. 4 illustrates the algorithm comprisedby the alternative embodiment. Column 1 illustrates the setting of theinitial transmission power prior to initializing AGC 108 calibration.Column 2 describes the AGC 108 calibration process and determineswhether there exists a need to increase transmission power. The upperportion of column 3 represents the normalization of the stored values bythe initial calibration measurements. The lower portion of column 3illustrates the algorithm for increasing transmission power. Each columnwill now be discussed in turn.

The process may begin in column 1 by obtaining a value for k (block402). The k value, denoting the number of times the AGC 108 gain settingis decremented, after which the transmission power is increased, may beprovided by the device manufacturer, an end-user, or any other suitableentity. The processor 150 may adjust the gain setting to the maximumgain setting (block 404). The transmitting transceiver 104A may adjustthe transmission power to the minimum power setting (block 406). Inblocks 408-414, the transmitting transceiver 104A then increments thetransmission power until the processor 150 determines that the signalpower at the ADCs 146 and 148 is at the top of the ADC dynamic signalpower range. Specifically, the transmitting transceiver 104A maytransmit a predetermined training transmission as described above (block408). The receiving transceiver 104B subsequently may receive, measureand store the channel estimation transmission (block 410). The receivingtransceiver 104B then may determine whether the signal power at the ADCs146 and 148 is at the top of the ADC 146 and 148 dynamic signal powerrange; that is, the receiving transceiver 104B may determine whether thesignal power at the ADCs is above a predetermined threshold (block 412).If the signal power at the ADCs 146 and 148 is not at a suitable level,the transmitting transceiver 104B may increase the transmission power arelatively small amount (block 414) and then may transmit anothertraining signal transmission (block 408). Otherwise, if the signal powerat the ADCs 146 and 148 is at an appropriate level, then thetransmission power is set correctly and may not need to be adjusted.

If the transmission power is properly set, then the actual AGCcalibration process may begin. The AGC calibration process, illustratedin column 2, may begin by initializing variables g=1, i=0 and j=0 (block416). As described above, g represents the combined correction factors,i represents the number of overall AGC gain settings decrementedthroughout the entire calibration process, and j represents the numberof AGC 108 gain settings decremented in the current range of k gainsettings. Counters i and j then may be incremented by 1 (block 418),starting an iterative AGC calibration loop. The transmitting transceiver104A may transmit an AGC calibration signal (block 420) and thereceiving transceiver 104B may receive and decode the raw measurement(block 422). The raw measurement may be taken from the output of the FFTmodule. The receiving transceiver 104B then may multiply the rawmeasurement value by the combined correction factor g to produce andstore a corrected measurement value (block 424).

The receiving transceiver 104B may determine whether AGC 108 gainsettings are at the minimum possible level, indicating that thecalibration process is complete (block 426). If the AGC 108 gainsettings are not at the minimum level, the receiving transceiver 104Bmay compare j and k values to determine whether the current range of kgain settings has been passed. For example, if k=10 and j=4, the currentrange of 10 gain settings has not been passed, indicating that thetransmission power is at a suitable level. The receiving transceiver104B may decrement the AGC 108 gain setting (block 430) and proceed torepeat the iterative loop by incrementing j and i (block 418). However,if j and k values are equal, indicating that the current range of k gainsettings has expired, the transmission power is insufficient to continuethe calibration and must be increased, as explained below.

If the AGC 108 gain settings are at the minimum possible level asdetermined by the receiving transceiver 104B (block 426), the AGCcalibration loop is complete. The receiving transceiver 104B maynormalize the stored measurements by dividing each measurement by thefirst measurement (block 432).

Column 3 illustrates the algorithm for increasing transmission power. Asdescribed above, control is passed from column 2 to column 3 if k gainsettings have passed and there exists a need to increase transmissionpower. The process may begin by incrementing the transmission power byan amount equal to the previous k AGC gain setting decrements (block434). For example, if after k decrements the AGC gain has been reducedby 8 dB, then the transmission power may be incremented by L=8 dB.Likewise, if k decrements result in an AGC gain reduction of 20 dB, thenthe transmission power may be incremented by L=20 dB. The transmittingtransceiver 104A then may transmit an AGC calibration signal (block 436)and the receiving transceiver 104B may receive and decode the rawmeasurement (block 438). The receiving transceiver 104B may compute thecorrected measurement by multiplying the raw measurement and thecorrection factor (block 440). The receiving transceiver 104Bsubsequently may compare the measurement values immediately before andimmediately after the transmission power adjustment to determine acorrection factor f and a combined correction factor g (block 442), aspreviously described. Once j is reset to 0 in order to keep track of thenumber of gain settings decremented within a range of k gain settings(block 444), the algorithm returns control to the calibration process ofcolumn 2 by decrementing the AGC gain setting (block 430) and repeatingthe calibration process for the decremented AGC gain setting.

As the AGC gain setting is decremented in the previous embodiment, thecombined correction factor g substantially increases. That is, as theAGC 108 approaches the lowest gain settings, g comprises severalcorrection factors f. Thus, as the AGC 108 approaches the lowest gainsettings, the algorithm is increasingly susceptible to larger errors. Tocompensate for such a problem, a third embodiment calibrates the AGC 108by beginning measurements midway through the AGC 108 gain setting rangeinstead of beginning measurements at the top of the AGC 108 gain settingrange. The third embodiment comprises a process that decrements the AGC108 gain setting for half of the gain setting range, resets the gainsetting to the midpoint, and increments the AGC 108 gain setting for theremaining half of the gain setting range.

In an example, an illustrative AGC 108 has a range of 80 dB with amidpoint value of −40 dB, a step size of 1 dB and a transmission powerincrease every 10 dB (k=10 steps, L=10 dB). Instead of beginning at 0 dBand stepping down to −80 dB as in previous embodiments, this processbegins at −40 dB and steps down a total of 40 dB, with transmissionpower increases every 10 dB. Upon completion, the process will havechanged transmission power only three times, thereby supplying onlythree f correction factors and minimizing the accumulation of error ing. To measure the remaining half of the 80 dB range, the AGC 108 may bereturned to the −40 dB mark and the gain settings may be incremented atotal of 40 dB, decrementing the transmit power every 10 dB. Thisalgorithm minimizes the number of f correction factors involved inconverting raw measurement data to corrected measurement data and thusminimizes error.

Although the foregoing embodiments are discussed in the context oflinear, equal steps, non-linear or unequal steps also may beaccommodated. The above discussion is meant to be illustrative of theprinciples and various embodiments of the present invention. Numerousvariations and modifications will become apparent to those skilled inthe art once the above disclosure is fully appreciated. It is intendedthat the following claims be interpreted to embrace all such variationsand modifications.

1. A method for calibrating an AGC in a MIMO-based system, the methodcomprising: transmitting a calibration signal; receiving the calibrationsignal; decoding the calibration signal to produce a measurement;storing the measurement; changing an AGC gain setting; and repeating thetransmitting, receiving, decoding, storing, and changing operations todetermine an optimal AGC gain setting.
 2. The method of claim 1, whereinthe transmitting, receiving, decoding, storing, and changing operationsare performed by a single multiple-input-multiple-output (MIMO) wirelessdevice.
 3. The method of claim 1, wherein the transmitting, receiving,decoding, storing, and changing operations are performed for each AGCgain setting.
 4. The method of claim 1, wherein transmitting saidcalibration signal comprises transmitting a single frequency centered ona fast Fourier transformer bin.
 5. The method of claim 1, furthercomprising generating said calibration signal by applying a non-zerocoefficient to an inverse fast Fourier transformer.
 6. The method ofclaim 1, wherein decoding the calibration signal comprises using a fastFourier transformer.
 7. The method of claim 1, further comprisingaccessing the measurement to improve AGC performance.
 8. The method ofclaim 1, further comprising normalizing the measurement.
 9. The methodof claim 1, wherein transmitting said calibration signal comprisesprepending a cyclic prefix to the calibration signal.
 10. A MultipleInput Multiple Output (MIMO)-based system, comprising: an adjustablegain amplifier that receives a wireless transmission comprising acalibration signal; and a digital processing device that comprises anautomatic gain control (AGC) coupled to the adjustable gain amplifier,the AGC controls a gain of the adjustable gain amplifier; wherein theAGC iteratively changes the gain of the adjustable gain amplifier toeach of a plurality of gain settings; and wherein the digital processinglogic iteratively decodes the calibration signal for each of theplurality of gain settings to produce a plurality of measurements thatare stored and used to identify a target AGC gain setting.
 11. TheMIMO-based system of claim 10, wherein the calibration signal comprisesa single frequency centered on a fast Fourier transformer bin.
 12. TheMIMO-based system of claim 10, further comprising a transmitter thatgenerates the calibration signal.
 13. The MIMO-based system of claim 12,wherein the calibration signal is generated by applying a non-zerocoefficient to an inverse fast Fourier transformer.
 14. The MIMO-basedsystem of claim 10, wherein the digital processing logic decodes thecalibration signal using a fast Fourier transformer.
 15. The MIMO-basedsystem of claim 10, wherein the digital processing logic furtheraccesses the plurality of measurements to improve AGC performance. 16.The MIMO-based system of claim 10, wherein the digital processing logicfurther normalizes the plurality of measurements.
 17. The MIMO-basedsystem of claim 10, wherein the calibration signal comprises a prependedcyclic prefix.
 18. A Multiple Input Multiple Output (MIMO)-based system,comprising: means for receiving a calibration signal; means for decodingthe calibration signal to produce a plurality of measurements, and forprocessing the produced plurality of measurements; and means forchanging a gain of the means for receiving; wherein the means forchanging iteratively changes the gain of the means for receiving to eachof a plurality of gain settings; and wherein the means for decoding andfor processing decodes the calibration signal for each of the pluralityof gain settings to produce the plurality of measurements that are usedto identify a desired gain setting.
 19. The MIMO-based system of claim18, further comprising means for transmitting the calibration signal.20. The MIMO-based system of claim 18, further comprising means forstoring the measurement.
 21. The MIMO-based system of claim 18, whereinthe means for decoding and for processing decodes the calibration signalusing a fast Fourier transformer.
 22. The MIMO-based system of claim 18,wherein the means for decoding and for processing further accesses theplurality of measurements to improve AGC performance.
 23. The MIMO-basedsystem of claim 18, wherein the means for decoding and for processingfurther normalizes the plurality of measurements.